Oilfield drilling fluid, often called “mud,” serves multiple purposes in the industry. Among its any functions, the drilling mud acts as a lubricant to cool rotary drill bits and facilitate faster cutting rates. The mud is mixed at the surface and pumped downhole through a bore of the drillstring to the drill bit where it exits through various nozzles and ports, lubricating and cooling the drill bit. After exiting through the nozzles, the “spent” fluid returns to the surface through an annulus formed between the drillstring and the drilled wellbore.
Furthermore, drilling mud provides a column of hydrostatic pressure, or head, to prevent “blow out” of the well being drilled. This hydrostatic pressure offsets formation pressures thereby preventing fluids from blowing out if pressurized deposits in the formation are breached. Two factors contributing to the hydrostatic pressure of the drilling mud column are the height (or depth) of the column (i.e., the vertical distance from the surface to the bottom of the wellbore) and the density (or its inverse, specific gravity) of the fluid used. Various weighting and lubrication agents are mixed into the drilling mud to obtain the right mixture for the type and construction of the formation to be drilled. Because the mud evaluation and mixture process is time consuming and expensive, drillers and service companies prefer to reclaim the returned drilling mud and recycle it for continued use.
Another significant purpose of the drilling mud is to carry the cuttings away from the drill bit to the surface. As a drill bit pulverizes or scrapes the rock formation at the bottom of the borehole, small pieces of solid material are left behind. The drilling fluid exiting the nozzles at the bit stir up and carry the solid particles of rock and formation to the surface within the annulus between the drillstring and the borehole. Therefore the fluid exiting the borehole from the annulus is a slurry of formation cuttings in drilling mud, and the cutting particulates must be removed before the mud can be recycled.
One type of apparatus used to remove cuttings and other solid particulates from drilling mud is commonly referred to in the industry as a “shale shaker” or “vibratory separator.” A shale shaker is a vibrating sieve-like table upon which returning used drilling mud is deposited and through which substantially cleaner drilling mud emerges. Typically, the shale shaker is an angled table with a generally perforated filter screen bottom. Returning drilling mud is deposited at a first end of the shale shaker. As the drilling mud travels across the perforated screen, the fluid falls through the perforations to a reservoir below thereby leaving the solid particulate material behind. The combination of the angle of inclination with the vibrating action of the shale shaker table enables the solid particles left behind to flow until they fall of the second end of the shaker table. The amount of vibration and the angle of inclination are typically adjustable to accommodate various drilling mud flow rates and particulate percentages in the drilling mud. After the fluid passes through the perforated bottom of the shale shaker, it can either return to service in the borehole immediately, be stored for measurement and evaluation, or it may pass through another, smaller size shale shaker or other equipment to further remove smaller cuttings.
As mud is circulated through the shaker separators and other cuttings removal apparatus, the flow rate of the feed mud may increase when the driller is flushing the wellbore or the geology of the wellbore requires a change in drilling fluid properties. The flow rate may increase to such an extent that the mesh of the screening surface can become congested with solids that are not removed fast enough to allow the fluid component of the feed mud to flow through the screen.
To prevent the loss of valuable drilling mud over the front edge of the screening surface and into the cuttings collection area during such flow rate increases, the front end of the separator is often raised so that the front edge of the screening surface is higher than the back edge of the screening surface. When the separator is actuated, the screening surfaces and basket within which they are secured vibrate at a desired frequency and with a predetermined motion, such as linear, elliptical, or circular. While the basket and screens vibrate at a predetermined frequency and motion, the housing to which the basket is resiliently mounted does not vibrate. This often results in the presence of a small gap between the back edge of the basket and the housing. Unfiltered drilling fluid drains directly into the filtered fluid collection area through the resulting gap. When the front end of the separator is raised, as during normal operation of the vibratory separator, the quantity of unfiltered drilling fluid that drains into the filtered fluid can increase when the mud depth increases. It would be an improvement to the quality of drilling fluid being filtered by the vibratory separator, to direct such unfiltered fluid into a secondary screen.
The condition of the screens may also contribute to the commingling of unfiltered drilling fluid with filtered drilling fluid. As drilling fluid solids are filtered from the drilling fluid, the wires making up the screening surface are subject to breakage. Such breakage is more prevalent near the back of the screening surface, where the unfiltered drilling mud is initially directed onto the screening surface. As the wires break, a hole or tear in the screen forms and becomes larger, which leads to more solids passing through the screening surface. Because separators are typically in continuous use, any repair operations and associated downtimes are minimized to the extent possible. Often, the screens of separators, through which the solids are separated from the drilling mud wear out over time and need replacement. Therefore, separator filter screens are typically constructed to be quickly and easily removed and replaced. Generally, through the loosening of only a few bolts, the filter screen can be lifted out of the shaker assembly and replaced within a matter of minutes. Additional screening surfaces that are vertically arranged would expose the drilling fluid to multiple screens, thereby reducing the effects of a tear or break in any single screen. Further, the replacement of a single screen could be deferred until additional screens break or there is down time in the drilling operation.
Many separator are equipped with screens having a lower mesh size than is preferable. This results in the removal of coarse solids but permits some solids that are larger than preferable to pass through the screen and remain in the filtered drilling fluid. Among the reasons for using screens having larger perforations include the desire to salvage drilling fluid when there are surges in the fluid flow to the shaker separator. By having larger perforations in the screen, more fluid passes and less fluid is discarded with the cuttings. Another reason for using such screens is to increase the capacity of the separator to filter the drilling fluid. Screens having smaller perforations cannot filter the same quantity of drilling fluid in a period of time as do screens having larger perforations. It would be an improvement to provide more desirable mesh sizes without the undesirable side effect of losing drilling fluid into the cuttings collection area.
Power loss or fluctuation in power to a shaker reduces the G-force of the shaker, causing the screening process to lose efficiency. The solids in the unfiltered mud are no longer propelled to the front of the screen and instead accumulate on the screening surface. As the used mud continues to be provided onto the screening surface, the lack of vibration results in the larger solids settling on the screening surface, causing it to become clogged such that much of the fluid does not pass through the screen mesh. If sufficient power is not promptly restored, the unfiltered drilling fluid will accumulate in the space defined by the inclined screen surface, the rear wall of the basket, the side walls of the separator to eventually overflow the front edge of the screening surface into a cuttings box or solids collection area. When the drilling fluid overflows into the cuttings collection area, additional treatment of the cuttings is usually required before the cuttings can be properly disposed. Although it is not desirable to have unfiltered drilling fluid continually released to the collection area for filtered drilling fluid, such a result is preferred when compared to losing the valuable drilling fluid in the cuttings collection area. More preferential would be to separately collect the unfiltered drilling fluid for re-circulation through the shaker.